1. Field of the Invention
The present invention relates to a sublimation-type thermal image transfer recording medium, and more particularly to a sublimation-type thermal image transfer recording medium capable of forming images with high image density on a transparent image-receiving film such as an OHP film for use with an overhead projector, and capable of performing multiply printing operations.
2. Discussion of Background
The optical path for forming a transmission-type image, such as an image made through a transparent film used for an over head projector (OHP) is a half the optical path for forming a reflection-type image. Therefore, even if such a transmission-type image is formed by use of a conventional image transfer recording medium for forming the reflection-type image, the obtained image density is insufficient for use in practice.
Under such circumstances, various types of image transfer recording media capable of forming a transmission-type image with high density have been proposed, for example, by the following improvements:
(1) The total amount of a dye contained in an ink layer of an image transfer recording medium is increased. In this kind of recording medium, as a matter of course, the thickness of the ink layer increases, which results in a decrease in the thermal sensitivity. PA1 (2) The concentration of a dye in an ink layer of an image transfer recording medium is increased. In this kind of recording medium, although the thickness of the ink layer does not substantially increase, the amount of a binder agent contained in the ink layer is relatively decreased. As a result, the mechanical strength of the ink layer is decreased and the adhesion between the ink layer and a support is reduced. The ink layer is therefore readily peeled off the support and is inappropriately transferred to an image-receiving medium. In addition to the above, crystallization of the dye contained in the ink layer is induced and bleeding of the dye to the surface of the ink layer takes place. These have serious adverse effects on the preservability of the obtained images. PA1 (3) A low-softening point material is added to an ink layer of an image transfer recording medium. In this kind of recording medium, due to the addition of a low-softening point material to the ink layer, the mechanical strength of the ink layer is decreased in particular during the printing operation. Consequently, the ink layer is entirely or partially peeled off the support or inappropriately transferred to the image-receiving medium in the course of printing operation. Furthermore, the preservability of the ink layer itself is reduced.
A thermal image transfer recording media for use in multiple printing has also been proposed, but has the drawbacks as mentioned in the above proposals (1) and (2).